During the drilling of an oil well, a usually aqueous fluid is injected into the well through the drill pipe and recirculated to the surface in the annular area between the wellbore wall and the drill string. The functions of the drilling fluid include: lubrication of the drill bit, transportation of cuttings to the surface, overbalancing formation pressure to prevent an influx of oil, gas or water into the well, maintenance of hole stability until casings can be set, suspension of solids when the fluid is not being circulated, and minimizing fluid loss into and possible associated damage/instability to the formation through which drilling is taking place.
Proper overbalancing of formation pressure is obtained by establishing fluid density at the desired level usually via the addition of barite. Transportation of cuttings and their suspension when the fluid is not circulating is related to the fluid viscosity and thixotropy which depend on solids content and/or use of a polymer. Fluid loss control is obtained also by the use of clays and/or added polymers.
Fluid properties are constantly monitored during the drilling operations and tailored to accommodate the nature of the formation stratum being encountered at the time. When drilling reaches the producing formation special concern is exercised. Preferentially, low solids content fluids are used to minimize possible productivity loss by solids plugging. Proper fluid density for overbalancing formation pressure may be obtained by using high salt concentration aqueous brines while viscosity and fluid loss control generally are attempted by polymer addition, and/or soluble particulates such as calcium carbonate or size salt in saturated brine solution.
When high permeability and/or poorly consolidated formations are penetrated as the zone of interest, a technique referred to as "under-reaming" often is employed in the drilling operations. In this process, the wellbore is drilled through the hydrocarbon bearing zone using conventional techniques and drilling muds. A casing generally is set in the well bore to a point just above hydrocarbon bearing zone. The hydrocarbon bearing zone then is redrilled using an expandable bit that increases the diameter of the hole. The purpose of the under-reaming is to remove damage from the permeable formation introduced during the initial drilling process by particles of the drilling mud and to increase the exposed surface area of the wellbore. Typically, under-reaming is effected utilizing special "clean" drilling fluids to minimize further formation damage. The high permeability of many hydrocarbon zones allows large quantities of the clean drilling fluid to be lost to the formation. Typical fluids utilized in under-reaming comprise expensive, aqueous, dense brines which are viscosified with a gelled or crosslinked polymer to aid in the removal of the drill cuttings. Such dense brines have been reported as being difficult to unload from formations once losses have occurred. Calcium and zinc-bromide brines can form highly stable, acid-insoluble compounds when reacted with some formation brines. Because of the high density of these brines, stratification tends to further inhibit the removal. The most effective means of preventing this type of formation damage is to limit completion brine losses to the formation.
Providing effective fluid loss control without damaging formation permeability in completion operations has been a prime requirement for an ideal fluid loss-control pill. Conventional fluid loss control pills include oil-soluble resins, calcium carbonate, and graded salt fluid loss additives have been used with varying degrees of fluid loss control. These pills achieve their fluid loss control from the presence of solvent-specific solids that rely on filter-cake build up on the face of the formation to inhibit flow into and through the formation. However, these additive materials can cause severe damage to near-wellbore areas after their application. This damage can significantly reduce production levels if the formation permeabilities is not restored to its original level. Further, at a suitable point in the completion operation, the filter cake must be removed to restore the formation's permeability, preferably to its original level.
A major disadvantage of using these conventional fluid loss additives is the long periods of clean up required after their use. Fluid circulation, which in some cases may not be achieved, is often required to provide a high driving force, which allows diffusion to take place to help dissolve the concentrated build up of materials. Graded salt particulates can be removed by circulating unsaturated salt brine to dissolve the particles. In the case of a gravel pack operation, if this occurs before gravel packing, the circulating fluid often causes sloughing of the formation into the wellbore and yet further loss of fluids to the formation. If removal is attempted after the gravel pack, the gravel packing material often traps the particles against the formation and make removal much more difficult. Other particulates, such as the carbonates can be removed with circulation of acid, however, the same problems may arise. Oil-soluble resins, carbonate and graded salt particulate will remain isolated in the pores of the formation unless they are in contact with solvent. In the cases where the solid material cover a long section of wellbore, the rapid dissolution by solvent causes localized removal. Consequently, a thief zone forms and the majority of the solvent leaks through the thief zone instead of spreading over the entire wellbore length.
The use of conventional gel pills such as linear viscoelastic or heavy metal-crosslinked polymers in controlling fluid loss requires pumping the material through large-diameter tubing because of high friction pressures. These materials are typically prepared at the well site.
Among the linear polymers used to form fluid loss control pills is hydroxyethylcellulose (HEC). HEC is generally accepted as a polymer fluid affording minimal permeability damage during completion operations. Normally, HEC polymer solutions do not form rigid gels, but control fluid loss by a viscosity-regulated mechanism. Such polymer fluids may penetrate deeper into the formation than crosslinked polymers. Permeability damage may increase with increasing penetration of such viscous fluids.
According to conventional wisdom, in high permeability reservoirs, a highly crosslinked gel is needed to achieve good fluid loss control. Though HEC is known for its low residue content, it is difficult to crosslink, particularly in regards to on-site or in situ formulations. However, according to M. E. Blauch et al. in SPE 19752, "Fluid Loss Control Using Crosslinkable HEC in High-Permeability Offshore Flexure Trend Completions," pages 465-476 (1989), while there are chemical methods to crosslink standard HEC, these methods have generally been found to be inapplicable to most completion practices.
Therefore, much effort has been expended to modify HEC to make it more easily crosslinkable, which adds to the expense and in some cases complexity of such systems. U.S. Pat. No. 4,552,215 to Almond et al. discloses a cellulose ether which is chemically modified to incorporate pendant vicinal dehydroxy groups which assume or can assume cis geometry. These modified celluloses can be crosslinked by zirconium (IV) metal ions and are useful for fluid loss control.
In SPE 29525, "A New Environmentally Safe Crosslinked Polymer for Fluid loss Control," pages 743-753 (1995), R. C. Cole et al. disclosed a polymer which has been prepared by grafting crosslinkable sites onto an HEC backbone. The polymer can be transformed into a rigid, internally crosslinked gel if the pH of the solution is adjusted from acidic to slightly basic through the use of non-toxic metal oxide crosslinker. No divalent or trivalent metals are associated with the polymer or included in its crosslinking chemistry. The crosslinking is effected by the use of a slowly soluble, non-toxic metal oxide. The resulting crosslink fluid is said to demonstrate shear-thinning and rehealing properties that provide for easy pumping. The rehealed gel is said to provide good fluid loss control upon placement. The polymer is referred to as a double-derivatized HEC (DDHEC). Instead of being a dry polymer in a bag, the DDHEC is a dispersion in an environmentally safe, non-aqueous, low-viscosity carrier fluid. The non-flammable carrier fluid is initially soluble in most brines. Hydration occurs only at specific, highly acidic conditions. At near neutral pH, the DDHEC polymer is dispersed into the mixing brine. When required, the pH is lowered, encouraging hydration to rapidly occur.
In SPE 36676, "Development and Field Application of a New Fluid Loss Control Material," pages 933-941 (1996), P. D. Nguyen et al. disclosed grating crosslinked, derivatized hydroxyethylcellulose (DHEC) into small particulates kept in a brine solution. Details of the chemistry and properties of the ungrated crosslinked DHEC were described in SPE 29525 discussed above. In SPE 36676, crosslinked DHEC was placed in a pressure chamber to which a perforated disk, cylinder or screen was attached to its end. Air was introduced at the other end of the pressure chamber to push the crosslinked material into and through the grating device and shredded. The shredded material is provided as a slurry concentrate and is said to be stable enough to store in this form. The slurry concentrate is then dispersed in a completion fluid.
U.S. Pat. No. 5,372,732 to Harris et al. discloses a dry, granulated, delayed crosslinking agent for use as a blocking gel in a workover operation comprising a borate source and a water-soluble polysaccharide comprising at least one member selected from the group of guar gum, hydroxypropylguar and carboxymiethylhydroxypropylguar. The blocking gel forms a relatively impermeable barrier cordoning off the production zone from the area undergoing the workover operation. The crosslinking agent is prepared by dissolving one of the water-soluble polysaccharides identified above in an aqueous solution. To the aqueous solution is added a borate source to form a crosslinked polysaccharide. The borate-crosslink polysaccharide is then dried and granulated. The delayed crosslinking agent is admixed with an aqueous gel containing a second-water soluble polysaccharide solution. As is well known in the art, the borate crosslink is a reversible crosslink in that the boratelpolymer crosslink at basic pH is in equilibrium with the borate ion and polymer crosslink sites (i.e., cis oriented hydroxyl groups), wherein the borate ion detaches from one site and then reattaches to another or the same site of the same or different polymer. Such crosslinked polymers are said to be self-healing since if the crosslink is broken it will reform at the same or different location. However, it is also known that HEC is not crosslinkable with borates. This is one reason why HEC has been derivatized by others to incorporate hydroxyl groups which can be in a cis orientation relative to one another.
Thus, there is a need to be able to use unmodified HEC in fluid loss control situations to thereby avoid the cost associated with derivatizing HEC for use in such systems. There is also a need to reduce the complexity of such systems for ease of use at a field site, preferably without the use of a chemist to prepare the fluid loss control pill composition. Further, there is also a need for a reliable viscous fluid loss control system containing no conventional fluid loss control solids.